I would guess that fireworks are more of a single game attraction - get the interest of the casual family or group of people who will go to the game because of the fireworks. The season ticket holder is assumed to be a more serious fan, or at least, is purchasing for business purposes. I highly doubt that the majority of season ticket holders made the decision to make that kind of investment based on the number of fireworks or bobblehead giveaways.

Besides, fireworks are for everyone and bobbleheads are only for the 10,000 people who show up at 10AM. I barely got the Frank Thomas one, there were 20 or so left at my gate at 11:30.

I wouldn't mind more lesser promotions like hats, t-shirts or pins since they're less expensive to make and more fans get end up getting them.